Buck is Dally's rodeo partner. He is responsible for getting Dally his job as a jockey for the Slash J. Dally often stays at Buck's house when he needs a place to sleep.
Buck made most of his money on bootlegging. He was a "tall lanky cowboy in his mid-twenties" with blond hair and used to have buckteeth "before he had the front two knocked out in a fight" (in Ponyboy's words).
The night that Johnny kills Bob in the park near the fountain, Buck hosts a party at his place. Johnny and Ponyboy knock on the door sometime after 2 A.M. - they ask for Dally. Merrill initially states that Dally is busy, but Johnny says to say that it's Ponyboy and Johnny. Merrill leaves, and they see him bringing Dally through the window. Later on he lets Dally use his T-Bird when he's visiting Johnny and Pony in the abandoned church. From there he has essentially no further involvement in the story.